When using sights for hunting and other field application there may be a conflict between speed and precision, speed being the time period between spotting the target, with the rifle lowered, and firing the first round. This speed is particularly important when shooting at targets in short ranges, where e.g. a game may appear and disappear within few seconds of time. Generally, the highest speed is achieved when sights not including any magnifying optics, which need to have a certain eye relief, are used, since the eye then will not have to find the virtual image of a reticle and a target, and superimpose the one on the other before the first round is fired. Magnifying optics also have restrictions regarding field of view.
Often the price paid for the speed is precision. In order to obtain better precision, in long distance shooting (e.g. >300 m) in particular, a telescopic sight may be preferred, e.g. with a magnification of three times (3×) or with a variable magnification or zoom (3-6×). A telescopic sight is defined by some characteristic parameters, such as the field of view, entrance pupil, exit pupil, eye relief, magnification, etc. These features are interconnected, such that if e.g. the eye relief is incorrect (not the correct distance between the eye and the eyepiece lens) the field of view will suffer. Magnifying sights, or telescopic sights, are well known, having a fixed magnification or a zoom function. Further details of telescopic sights will not be discussed here.
The telescopic sight is inferior in terms of speed, since the aiming eye has to find the exit pupil, find the target through the telescope and superimpose the reticle image on the target, and then fire a round. Further, when using a magnifying telescopic sight only one eye, the aiming eye, can be open, which further slows the speed of target acquisition. Having both eyes open and receiving a non-magnified image in one and a magnified in the other generally is too much information for the visual organs to handle.
Manufacturers of sights have strived to provide a sight with the benefits of both of the above sights, and a known combination includes a telescopic sight having an iron sight on its upper side, see e.g. U.S. Pat. No. 4,841,659. There are other known combinations where two optical sights are arranged side-by-side (or on top of each other), combined in one casing or not.